Coatings and materials can become soiled from debris (particles, insects, oils, etc.) impacting the surface. The debris affects airflow over the surface as well as aesthetics and normally is removed by washing.
Many attempts are described to mitigate insect accumulation during the early days of aircraft development. These include mechanical scrapers, deflectors, traps, in-flight detachable surfaces, in-flight dissolvable surfaces, viscous surface fluids, continuous washing fluids, and suction slots. The results of most of these trials were determined ineffective or impractical for commercial use.
Recently, Wohl et al., “Evaluation of commercially available materials to mitigate insect residue adhesion on wing leading edge surfaces,” Progress in Organic Coatings 76 (2013) 42-50 describe work at NASA to create anti-insect adhesion surfaces. Wohl et al. tested the effect of organic-based coatings on insect adhesion to surfaces, but the coatings did not fully mitigate the issue. Wohl et al. also describe previously used approaches to reduce bug adhesion such as mechanical scrapers, deflectors, paper and/or other coverings, elastic surfaces, soluble films, and washing the surface continually with fluid.
One approach is to create a self-cleaning surface that removes debris from itself by controlling chemical interactions between the debris and the surface. Enzyme-filled coatings leech out enzymes that dissolve debris on the surface. However, the enzymes are quickly depleted and cannot be refilled, rendering this approach impractical. Fluorofluid-filled surfaces have very low adhesion between impacting debris and the surface. However, if any of the fluid is lost, the surface cannot be refilled/renewed once applied on the vehicle, and thus loses its properties (see Wong et al., “Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity,” Nature 477, 443-447, 2011).
Superhydrophobic and superoleophobic surfaces create very high contact angles (>150°) between the surface and drops of water and oil, respectively. The high contact angles result in the drops rolling off the surface rather than remaining on the surface. These surfaces do not repel solid foreign matter or vapors of contaminants. Once soiled by impact, debris will remain on the surface and render it ineffective. Also, these surfaces lose function if the nanostructured top surface is scratched.
Kok et al., “Influence of surface characteristics on insect residue adhesion to aircraft leading edge surfaces,” Progress in Organic Coatings 76 (2013) 1567-1575, describe various polymer, sol-gel, and superhydrophobic coatings tested for reduced insect adhesion after impact. The best-performing materials were high-roughness, superhydrophobic surfaces. However, they did not show that debris could be removed from the superhydrophobic surfaces once insects broke on the surface.
Fluoropolymer sheets or treated surfaces have low surface energies and thus low adhesion force between foreign matter and the surface. However, friction between impacting debris and the surface results in the sticking of contaminants.
Polymeric materials having low surface energies are widely used for non-stick coatings. These materials are tailored with careful control of their chemical composition (thus surface energy) and mechanical properties. Polymers containing low-energy perfluoropolyethers and perfluoroalkyl groups have been explored for low adhesion and solvent repellency applications. While these low-energy polymers facilitate release of materials adhering to them in both air and water, they do not necessarily provide a lubricated surface to promote clearance foreign substances. See Vaidya and Chaudhury, “Synthesis and Surface Properties of Environmentally Responsive Segmented Polyurethanes,” Journal of Colloid and Interface Science 249, 235-245 (2002).
A fluorinated polyurethane is described in U.S. Pat. No. 5,332,798 issued Jul. 26, 1994 to Ferreri et al. U.S. Pat. No. 4,777,224 to Gorzynski et al. describes the process for the production of anionic polyurethanes comprising aliphatic dihydroxy compounds (greater than 10 carbon atoms), an aliphatic diol carrying an acid group, and a polyether. The polyurethane aqueous solutions are used for coating and sizing of paper. U.S. Pat. No. 4,956,438 to Ruetman et al. describes the composition and preparation of polyurethane ionomers synthesized by making an ionic prepolymer and chain-extending it with a polyol requiring three or more reactive hydroxyl groups. U.S. Pat. No. 7,655,310 to Trombetta describes polyurethanes containing perfluoropolyethers with ionizable groups such as a carboxylic acid or amine functionality for making waterborne systems. The patent also describes the use of pendant silanes (i.e. trimethoxysilane groups) for crosslinking the network. U.S. Pat. No. 6,992,132 to Trombetta et al. describes an aqueous dispersion of a linear crosslinkable ionomeric polyurethane containing carboxylic groups and having a perfluoropolyether structure and a crosslinking agent.
In view of the shortcomings in the art, improved coating materials and material systems, and compositions suitable for these coating systems, are needed. Improvement in lubrication or decrease in the coefficient of friction would better enable material to be cleared from a surface using the natural motion of an automotive or aerospace vehicle.